1. Field of the Invention
The present invention relates to a millimeter wave reception device, a mounting structure for a millimeter wave reception device, and a millimeter wave transmission/reception device. In particular, the present invention relates to a millimeter wave reception device, a mounting structure for a millimeter wave reception device, and a millimeter wave transmission/reception device installed outdoors.
2. Description of the Background Art
Japanese Patent Laying-Open Nos. 2009-017581 and 2005-005980 are prior documents disclosing a television community reception system and a transmission/reception device utilizing a millimeter wave communication system. Since broadcast signals are wirelessly distributed to households in the television community reception system and the transmission/reception device described in Japanese Patent Laying-Open Nos. 2009-017581 and 2005-005980, there is no need to provide a distributor and a long coaxial cable for signal transmission.
FIG. 11 is a view schematically showing a mounting structure for a conventional millimeter wave transmission/reception device. As shown in FIG. 11, in a conventional millimeter wave transmission/reception device 33, a millimeter wave transmission device 31 is arranged on a lower surface of a tip of an arm 34 provided to protrude laterally from a roof of a building 38. Millimeter wave transmission device 31 is set to transmit radio waves 37 downward. Radio waves 37 transmitted from millimeter wave transmission device 31 have directivity, and propagate downward while spreading slightly as shown in FIG. 11.
On the other hand, a millimeter wave reception device 32 is arranged on an upper surface of a tip of an arm 35 provided to protrude laterally from a balcony 36 of each floor. A plurality of millimeter wave reception devices 32 are each arranged to face millimeter wave transmission device 31. Millimeter wave reception device 32 is provided with an antenna portion receiving radio waves 37.
The antenna portions of the plurality of millimeter wave reception devices 32 are arranged in a region in which radio waves 37 reach, and are arranged such that, when the antenna portions are projected in a direction in which radio waves 37 are transmitted, the antenna portions do not overlap each other. As the antenna portion, a planar antenna, a lens antenna, a horn antenna, or the like is used. In millimeter wave reception device 32 installed outdoors, the antenna portion is housed within a hemispherical cover made of a material having weather resistance to suppress deterioration of the antenna portion.
FIG. 12A is a top view schematically showing a structure of a conventional millimeter wave reception device. FIG. 12B is a cross sectional view schematically showing the structure of the conventional millimeter wave reception device. In FIG. 12B, for convenience of explanation, millimeter wave transmission device 31 is also shown schematically.
As shown in FIGS. 12A and 12B, in conventional millimeter wave reception device 32, a lens antenna 39 is arranged on an upper surface of a base member 43. A hemispherical cover 40 is provided above lens antenna 39. An output terminal 41 is provided on a side surface of base member 43. Output terminal 41 is connected to lens antenna 39 through a circuit board 42a, an antenna-integrated circuit element 42b, and a circuit not shown.
An apex portion 39a and a central portion 39b of lens antenna 39 are arranged to be located on an identical straight line in a vertical direction. Apex portion 39a and central portion 39b of lens antenna 39 are arranged to be located within the region in which radio waves 37 transmitted from millimeter wave transmission device 31 reach. A region between arrows indicated by two broken lines shown in FIG. 12B represents the region in which radio waves 37 reach.
Since millimeter wave reception device 32 is installed outdoors, it is exposed to wind and rain. When rain sticks to or snow is accumulated on an upper surface of the antenna portion of millimeter wave reception device 32, reception propagation loss of radio waves 37 occurs at the antenna portion. When hemispherical cover 40 is mounted above the antenna portion, rain or snow failing on cover 40 is likely to accumulate in the vicinity of a top portion of cover 40 because the top portion of cover 40 has a low inclination.
Since the apex portion and the central portion of the antenna portion are located below the top portion of cover 40, accumulation of rain or snow in the vicinity of the top portion of cover 40 results in an increase in the reception propagation loss of radio waves 37 at the antenna portion, causing a situation where wireless transmission cannot be performed. In addition, when cover 40 has a gently sloping shape such as a hemispherical shape, a bird, an insect, or the like readily lands on cover 40. Also when a bird, an insect, or the like lands on cover 40, the reception propagation loss of radio waves 37 at the antenna portion is increased, causing a situation where wireless transmission cannot be performed.